The invention relates generally to an alarm system for providing visual and/or audio warnings and, more particularly, to an apparatus and a concomitant method for synchronizing a plurality of visual and/or audio alarm units.
This invention relates to circuits for electronic alarm systems such as are used to provide visual and audio warning in electronic fire alarm devices and other emergency warning devices and, more particularly, to a control circuit which enables the system to provide both a visual and an audio alarm signal, including a silence feature, while using only one signal wire loop.
Strobe lights and/or audio horns are used to provide warning of potential hazards or to draw attention to an event or activity. An important field of use for these signaling devices is in electronic fire alarm systems. Strobe alarm circuits typically include a flashtube and a trigger circuit for initiating firing of the flashtube, with energy for the flash typically supplied from a capacitor connected in shunt with the flashtube. In some known systems, the flash occurs when the voltage across the flash unit (i.e., the flashtube and associated trigger circuit) exceeds the threshold voltage required to actuate the trigger circuit, and in others the flash is triggered by a timing circuit. After the flashtube is triggered, it becomes conductive and rapidly discharges the stored energy from the shunt capacitor until the voltage across the flashtube has decreased to a value at which the flashtube is extinguished and becomes non-conductive.
In a typical alarm system, a loop of several flash units is connected to a fire alarm control panel which includes a power supply for supplying power to all flash units in the loop when an alarm condition is present. Each unit typically fires independently of the others at a rate determined by its respective charging and triggering circuits. Underwriters Laboratories specifications require the flash rate of such visual signaling devices to be between 20 and 120 flashes per minute.
In addition to having a strobe alarm as described above, it may also be desirable to have an audio alarm signal to provide an additional means for alerting persons who may be in danger. In such systems, a xe2x80x9csilencexe2x80x9d feature is often available whereby, after a period of time has elapsed from the initial alarm, the audio signal may be silenced either automatically or manually. Heretofore, in a system where alarm units having both a visual alarm signal and an audio alarm signal have been implemented, two control loops, one for video and one for audio, have been required between the fire alarm control panel and the series of alarm units.
In a system as described above, the supply voltage may be 12 volts or 20-31 volts, and may be either D.C. supplied by a battery or a full-wave rectified voltage. Underwriters Laboratories specifications require that operation of the device must continue when the supply voltage drops to as much as 80% of nominal value and also when it rises to 110% of nominal value. However, when the voltage source is at 80% of nominal value, the strobe may lose some intensity which could prove crucial during a fire emergency.
Thus, it is desirable to provide a control circuit which will enable an alarm system to provide both audio and visual synchronized alarm signals using only a single control signal wire loop between the alarm units, while allowing for the capability of silencing the audio alarm.
It is also desirable to provide the ability to lower the flash frequency when a low input voltage is detected, thereby ensuring a proper flash brightness.
It is also desirable to provide an alarm interface circuit which will enable an existing alarm system to sound a Code 3 alarm whether or not the existing alarm system is already equipped with Code 3 capability.
It is also desirable to provide a circuit having these properties and which will also work with: (a) both D.C. and full-wave rectified supplies; (b) all fire alarm control panels; and (c) mixed alarm units (i.e., 110 candela and 15 candela with and without audio signals).
It is also desirable to provide a method of reducing the number of synchronization pulses transmitted to the alarm units, thereby increasing the reliability of the overall alarm system.
In accordance with the present invention, an alarm system is provided which includes a control circuit that allows multiple audio/visual alarm circuits, connected together by a single two-wire control loop, to be synchronously activated when an alarm condition is present. The control circuit also allows for other alarm control functions, such as the deactivation of the audio alarm, to be carried out using only the single control loop. The control circuit is able to provide these functions by interrupting power to the alarm units for approximately 10 to 30 milliseconds at a time. Preferably, each alarm unit is equipped with a microcontroller which is programmed to interpret the brief power interrupt, or xe2x80x9cdrop outxe2x80x9d, as either a synchronization signal or a function control signal, depending on the timing of the drop out. The microcontroller can also be programmed to interpret different sequences of drop outs as control signals for other functions such as reactivation of the audio alarm.
The alarm unit is capable of detecting a low input voltage. When the detected voltage drops below a predetermined threshold, the alarm unit will lower the frequency of the visual alarm signal, preferably a strobe, to ensure that the strobe flashtube receives enough energy to flash at an adequate brightness.
The alarm unit is also capable of functioning independently of any synchronization signal from the control circuit. In the event a synchronization signal is not received, an internal timer will cause the flashtube to flash at a predetermined rate.
Furthermore, the synchronization signal can be implemented as a reference or reset signal from which the alarm units derive a reference time to begin activation of the alarm units. Thus, when an alarm unit receives a reference synchronization signal, the alarm unit will use that reference synchronization signal as a reference point in time to trigger a series of flashes and/or audio tones.